There are three main problems associated with the deployment of sensors in harsh environments. Firstly, there are the practical difficulties in getting the sensor to the correct position in the first instance and then, possibly, retrieving it later. Secondly, either the sensor must have a means of storing information, or it must have a means of communicating between the sensor itself and the information processing system. Thirdly, the sensor must be robust enough, not only to take the rigours of deployment, but also be able to function successfully for significant periods of time under harsh conditions. These three aspects are of particular importance in the oil extraction industry. The measurement of pressure at the bottom of an oil well is important for the efficient extraction of the oil reserves. The industry takes two different approaches. In the one case, a measurement system is lowered down the production tube of the well to take and record measurements for a short period before being returned to the surface. Although convenient for use with existing wells and only requiring short term stability of calibration, this approach gives limited information and interferes with the production of oil. In the other case, a measurement system is permanently installed during the construction of the oil well. This has the advantage of giving continuous measurements without interfering with oil production but places very severe requirements on the measurement stability of the sensor and its ability to withstand the high pressures and temperatures in the well for many years. Should such a sensor fail, or doubts be cast upon its accuracy, then the current practice would be to abandon it since it would be too expensive to replace. The permanent installation of a measurement system at the bottom of the oil well also requires significant technical problems to be solved in transferring information via metal or optical fiber cables, or radio links, up to the surface.
The present invention describes a pressure sensor which may be either temporarily or permanently deployed in a convenient fashion, using existing technology familiar to industries such as the oil industry.
An aim of the present invention is to provide apparatus for the measurement in a hostile environment preferably at a remote location.
According to the present invention there is provided apparatus for measuring pressure, which apparatus comprises pressure communicating means for location at a point at which pressure is to be measured, a primary tube containing a fluid for linking the pressure communicating means to a control point where the pressure information is required, sealing means for sealing the primary tube so as to form a closed system when the pressure communicating means is closed, pressure control means for changing the pressure of the fluid within the primary tube, volume measuring means for measuring changes in the volume of the fluid within the primary tube as the pressure of the fluid is varied, pressure measuring means for measuring the pressure of the fluid at some point within the fluid, and remote from the control point, and transmitter means for transmitting pressure information from the pressure measuring means to the control point.
The pressure communicating means may be a device which may exist in a number of states according to the relative pressure difference between an external and an internal pressure, and whose volume depends upon this pressure difference. The external pressure is that which surrounds the pressure communicating means and is usually the pressure to be measured. The internal pressure is that of the fluid contained in the primary tube at the point of connection between the pressure communicating means and the primary tube. When the external pressure is higher than the internal pressure by more than a fixed limit, the pressure communicating means contains a fixed volume of the primary tube fluid which constitutes one state, the OFF state. Similarly; when the external pressure is lower than the internal pressure by more than a different fixed limit, the pressure communicating means contains a different fixed volume of the primary tube fluid, which constitutes a second state, the ON state. A third state, the ACTIVE state, occurs when the external pressure lies between these fixed pressure limits relative to the internal pressure. In which case, the volume of primary tube liquid contained by the communicating means depends upon the pressure difference in some consistent fashion and lies between the two volume limits. When the pressure communicating means takes this form, the whole apparatus is a sealed system with no direct contact between the fluid in the primary tube and the external environment.
Alternatively, the pressure communicating means may be a device which may exist in a number of states according to a control signal provided separately. In addition, the pressure communicating means may not contain a mechanical interface between the pressure in the primary tube and that in the region to be measured, and so there will be direct contact between the fluid in the primary tube and the external environment when the pressure valve is in the ACTIVE state.
The primary tube may be preferably narrow bore hydraulic tubing whose change in dimension with pressure is either small or well-known. The fluid contained by the primary tube may be a hydraulic oil, or any other fluid that is virtually incompressible or whose compressibility is well-known.
The sealing means may take any form that ensures that at all times the pressure within the primary tube may be adequately controlled both in accuracy and range. In particular, the pressure in the primary tube must be able to match the pressure to be found in the external environment to be measured, so that the pressure communicating means may operate as intended and the pressure be measurable.
The pressure control means may be any apparatus which permits the pressure to be changed from one value to another under some external control. Such apparatus could be a force acting on a piston inside a cylinder containing the-primary tube fluid.
The pressure measuring means may be any convenient apparatus which converts pressure into an indication such as a pressure dial gauge or a pressure transducer.
The volume measuring means may be any apparatus suitable for measuring the change in volume of the fluid within the primary tube and which has the sensitivity to detect the change in volume of the pressure communicating means as it changes state. An example of such apparatus would be the measurement of the position of a piston in a cylinder where the piston is used to determine the pressure in the primary tube fluid.
In one embodiment of the present invention, the means of changing the pressure, the volume measuring means, and the pressure measuring means, are all located together at the control point.
In a preferred embodiment of the apparatus, there is also provided means of measuring the temperature along the length of the primary tube. The temperature measuring means may be preferably an optical fiber distributed temperature sensor, either outside or inside the primary tube. An example of such a sensor is the York Sensors Limited DTS80 which is commercially available.
In another preferred embodiment of the apparatus, there is also provided pressure sensing means of such a dimension and structure that it it is placeable inside the primary tube, locating means for locating the pressure sensing means at any desired point within the primary tube, and transmitter means for transmitting the pressure information from the pressure sensing means to the control point. More preferably there is also provided means for removing the pressure sensing means. The pressure sensing means may be a passive optical fiber sensor and the transmitter means may be optical fiber cables. More preferably, the pressure-sensing means may be a passive optical fiber sensor such as the polarimetric interferometer optical fiber pressure and temperature sensor disclosed in PCT patent application No. PCT/GB/93/00315.
A further embodiment includes a secondary tube which connects the furthest point at which a pressure sensor may be located in the primary tube to the control point so that a return path for the fluid in the primary tube is provided. It is then possible to locate the pressure sensor by means of controlling the flow of fluid through the primary and secondary tubes, by, for example, a pump with the capability of reversible flow.
During operation, the pressure of the fluid in the primary tube is increased, and the volume monitored, until there is a change in volume equivalent to the change of a pressure valve from the OFF state to the ON state. The pressure is then held constant in the ACTIVE state between the two volume limits by the pressure control means at some defined point, preferably the mid-point. The pressure at the pressure communicating means may then be computed from (a) the measured pressure obtained from the pressure sensing means, (b) the physical separation of the location of the pressure communicating means and the point at which the pressure is measured using the effect of gravity on hydrostatic pressure and the effect of temperature upon density of the fluid, and (c) any residual correction needed to compensate for any pressure loss caused by activation of the pressure communicating means. In general, it is desirable to locate the pressure sensing means close to the pressure communicating means in order to minimize the required correction to the measured pressure. In order for the volume change arising from the change in states of the pressure communicating means to be detected, it is necessary for the compressibility of the fluid in the primary tube to be low or well-known, and for the change in volume of the primary tube and the connected volumes with pressure to be small or well-known.
If P(h) and p(h,T) are the pressure and density respectively, at height h and temperature T, then the pressures at two different heights a and b are related by well-known theory as follows:       P    ⁡          (      a      )        =            P      ⁡              (        b        )              -          g      ⁢                        ∫          a          b                ⁢                              p            ⁡                          (                              h                ,                T                            )                                ⁢                      ⅆ            h                              
where g is the gravitational constant.
Thus, in order to be able to compute the pressure at one height from measurements made at another, it is necessary to know the height difference as well as the density of the fluid as a function of height. Where the temperature of the fluid is approximately constant, sufficient accuracy may be achieved simply by assuming a uniform density of the liquid. However, where large changes in temperature occur, and where maximum accuracy is required, it will be necessary to measure the temperature of the fluid as a function of height and or minimize the height difference between the height at which the pressure is required and the height at which it is measured.
If the pressure difference between that pressure needed to put the pressure communicating means into the OFF state and that pressure needed to put it into the ON state is known, then it will be possible to calibrate the pressure sensitivity system by varying the pressure from the one state to the other and noting the volume change. This calibration may be then used to convert a measured volume change into a calculated pressure change when the pressure communicating means is held in the active region.
In another preferred embodiment, there are provided additional pressure communicating means connected at different points along the primary tube, and distinguishing means for distinguishing the state of a particular pressure communicating means as the pressure is changed. Such distinguishing means may be implicit, such as, for example, when the pressure communicating means are used to measure the pressure at a variety of different heights and where the pressure is known to be strongly correlated with height, then identical pressure communicating means may be used and the point at which the pressure is being measured deduced from the sequence of volume changes with pressure change. Another example where the distinguishing means are implicit is when pressure communicating means of different volumes are used so that the communicating means is uniquely identified by its volume change as the pressure is varied. For example, each pressure communicating means having twice the volume of its predecessor along the primary tube would be suitable.
In one preferred embodiment, there is provided one or more pressure sensing means located at each pressure communicating means so that it is not necessary to take into account the position of the pressure valve by computing a correction.
In another embodiment, there are provided more than one pressure sensing means positioned at different locations as convenient, but not necessarily adjacent to a pressure communicating means. For example, the temperature at a pressure valve may be too high to permit the location of the pressure sensing means at that point. However, a pressure sensing means at the control point may not be able to provide suitable response to rate of change in pressure at the pressure communicating means because the primary tube is of such a bore and such a length that its impedance restricts the rate of change in pressure. In which case, it would be advantageous to position a pressure sensing means at some intermediate point in the primary tube between the pressure communicating means and the control point.
In another embodiment the pressure communicating means is operated by separate means, either hydraulic or electric, so that the state of the communicating means may be controlled at will. This may be advantageous for both safety and operational purposes, but does require additional equipment. In addition, there may be no mechanical interface between the fluid in the primary tube and that in the external region to be measured when the pressure communicating means is in the ACTIVE state. This has the advantage that the properties of such an interface do not affect the pressure measurement, but has the disadvantage that there may be loss of fluid from the primary tube. If this were to occur then there would need to be provided means of ensuring that there was sufficient fluid available in the apparatus at all times for satisfactory performance.